1. Field of the Invention
The present invention relates to bicycle freewheel gear clusters, and more particularly, to an improvement on bicycle freewheel gear cluster gear-shifting mechanism that shifts the drive chain from one sprocket wheel to another.
2. Description of Prior Art
Conventional freewheel gear cluster at the rear of a bicycle includes a number of sprocket wheels provided about the same axis and having different diameters. A derailleur (a gear-shifting mechanism) is used to shift the drive chain of the bicycle from one sprocket wheel to another. In general, the drive chain is shifted to a sprocket wheel having a larger diameter (gear up) when the bicycle is climbing up a hill, and shifted to one having a smaller diameter (gear down) when the bicyclist wants to ride the bicycle at high speeds.
It is a drawback of the conventional freewheel gear cluster that when gearing up from a small-diameter sprocket wheel to an intermediate-diameter one, the drive chain may overshoot to and touches the rim of the large-diameter sprocket wheel (as illustrated by dotted lines in FIG. 6), thus causing the drive chain to rub against the rim of the large-diameter sprocket wheel and thereby generating noise. Worse, due to the overshoot, the chain wheel can hardly be quickly and precisely engaged with the intermediate-diameter sprocket wheel.
To solve the aforementioned problem, a prior art method is to provide protrusions on the rim of the intermediate-diameter sprocket wheel facing the small-diameter sprocket wheel. These protrusions allow the drive chain to be meshed on them when the drive chain is gearing up from the small-diameter sprocket wheel to the intermediate-diameter one. The large-diameter sprocket wheel is also provided with such protrusions allowing smooth gearing of the drive chain from the intermediate-diameter sprocket wheel up to it.
It is, however, a disadvantage of the prior art method that the drive chain may not be properly meshed on the target sprocket wheel if the bicyclist operates the derailleur improperly or if the drive chain is loose or the protrusions are worn out.
Still, when the drive chain is gearing down to a smaller sprocket wheel, the links of the chain can be caught by the protrusions, thus causing the drive chain not to run smoothly through the gear-down operation.